# Qemu COLO Fault Tolerance

This document gives an overview of COLO's design and how to use it.

## Background

Virtual machine (VM) replication is a well known technique for providing application-agnostic software-implemented hardware fault tolerance, also known as "non-stop service".

COLO (COarse-grained LOck-stepping) is a high availability solution. Both primary VM (PVM) and secondary VM (SVM) run in parallel. They receive the same request from client, and generate response in parallel too. If the response packets from PVM and SVM are identical, they are released immediately. Otherwise, a VM checkpoint (on demand) is conducted.

## Architecture

The architecture of COLO is shown in the diagram below. It consists of a pair of networked physical nodes: The primary node running the PVM, and the secondary node running the SVM to maintain a valid replica of the PVM. PVM and SVM execute in parallel and generate output of response packets for client requests according to the application semantics.

The incoming packets from the client or external network are received by the primary node, and then forwarded to the secondary node, so that both the PVM and the SVM are stimulated with the same requests.

COLO receives the outbound packets from both the PVM and SVM and compares them before allowing the output to be sent to clients.

The SVM is qualified as a valid replica of the PVM, as long as it generates identical responses to all client requests. Once the differences in the outputs are detected between the PVM and SVM, COLO withholds transmission of the outbound packets until it has successfully synchronized the PVM state to the SVM.

Overview:

      Primary Node                                                            Secondary Node
    +------------+  +-----------------------+       +------------------------+  +------------+
    |            |  |       HeartBeat       +<----->+       HeartBeat        |  |            |
    | Primary VM |  +-----------+-----------+       +-----------+------------+  |Secondary VM|
    |            |              |                               |               |            |
    |            |  +-----------|-----------+       +-----------|------------+  |            |
    |            |  |QEMU   +---v----+      |       |QEMU  +----v---+        |  |            |
    |            |  |       |Failover|      |       |      |Failover|        |  |            |
    |            |  |       +--------+      |       |      +--------+        |  |            |
    |            |  |   +---------------+   |       |   +---------------+    |  |            |
    |            |  |   | VM Checkpoint +-------------->+ VM Checkpoint |    |  |            |
    |            |  |   +---------------+   |       |   +---------------+    |  |            |
    |Requests<--------------------------\ /-----------------\ /--------------------->Requests|
    |            |  |                   ^ ^ |       |       | |              |  |            |
    |Responses+---------------------\ /-|-|------------\ /-------------------------+Responses|
    |            |  |               | | | | |       |  | |  | |              |  |            |
    |            |  | +-----------+ | | | | |       |  | |  | | +----------+ |  |            |
    |            |  | | COLO disk | | | | | |       |  | |  | | | COLO disk| |  |            |
    |            |  | |   Manager +---------------------------->| Manager  | |  |            |
    |            |  | ++----------+ v v | | |       |  | v  v | +---------++ |  |            |
    |            |  |  |+-----------+-+-+-++|       | ++-+--+-+---------+ |  |  |            |
    |            |  |  ||   COLO Proxy     ||       | |   COLO Proxy    | |  |  |            |
    |            |  |  || (compare packet  ||       | |(adjust sequence | |  |  |            |
    |            |  |  ||and mirror packet)||       | |    and ACK)     | |  |  |            |
    |            |  |  |+------------+---+-+|       | +-----------------+ |  |  |            |
    +------------+  +-----------------------+       +------------------------+  +------------+
    +------------+     |             |   |                                |     +------------+
    | VM Monitor |     |             |   |                                |     | VM Monitor |
    +------------+     |             |   |                                |     +------------+
    +---------------------------------------+       +----------------------------------------+
    |   Kernel         |             |   |  |       |   Kernel            |                  |
    +---------------------------------------+       +----------------------------------------+
                       |             |   |                                |
        +--------------v+  +---------v---+--+       +------------------+ +v-------------+
        |   Storage     |  |External Network|       | External Network | |   Storage    |
        +---------------+  +----------------+       +------------------+ +--------------+

### Components

You can see there are several components in COLO's diagram of architecture. Their functions are described below.

#### HeartBeat

Runs on both the primary and secondary nodes, to periodically check platform availability. When the primary node suffers a hardware fail-stop failure, the heartbeat stops responding, the secondary node will trigger a failover as soon as it determines the absence.

#### COLO disk Manager

When primary VM writes data into image, the colo disk manager captures this data and sends it to secondary VM's which makes sure the context of secondary VM's image is consistent with the context of primary VM 's image. For more details, please refer to docs/block-replication.txt.

#### Checkpoint/Failover Controller

Modifications of save/restore flow to realize continuous migration, to make sure the state of VM in Secondary side is always consistent with VM in Primary side.

#### COLO Proxy

Delivers packets to Primary and Secondary, and then compare the responses from both side. Then decide whether to start a checkpoint according to some rules. Please refer to docs/colo-proxy.txt for more information.

Note: HeartBeat has not been implemented yet, so you need to trigger failover process by using 'x-colo-lost-heartbeat' command.

### COLO operation status

Overview:

    +-----------------+
    |                 |
    |    Start COLO   |
    |                 |
    +--------+--------+
             |
             |  Main qmp command:
             |  migrate-set-capabilities with x-colo
             |  migrate
             |
             v
    +--------+--------+
    |                 |
    |  COLO running   |
    |                 |
    +--------+--------+
             |
             |  Main qmp command:
             |  x-colo-lost-heartbeat
             |  or
             |  some error happened
             v
    +--------+--------+
    |                 |  send qmp event:
    |  COLO failover  |  COLO_EXIT
    |                 |
    +-----------------+

COLO use the qmp command to switch and report operation status. The diagram just shows the main qmp command, you can get the detail in test procedure.

## Test procedure

### Setup

Here we are running both instances on the same host for testing, change the IP Addresses if you want to run it on two hosts. Initially `127.0.0.1` is the Primary Host and `127.0.0.2` is the Secondary Host.

COLO uses double the guest ram size on the secondary side. The Qemu version should be the same on both hosts.

### Startup qemu

1. Primary: Initially, `$imagefolder/primary.qcow2` needs to be copied to all hosts. You don't need to change any IP's here, because `0.0.0.0` listens on any interface. The chardev's with `127.0.0.1` IP's loopback to the local qemu instance:

    # imagefolder="/mnt/vms/colo-test-primary"

    # qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
       -device piix3-usb-uhci -device usb-tablet -name primary \
       -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
       -device rtl8139,id=e0,netdev=hn0 \
       -chardev socket,id=mirror0,host=0.0.0.0,port=9003,server=on,wait=off \
       -chardev socket,id=compare1,host=0.0.0.0,port=9004,server=on,wait=on \
       -chardev socket,id=compare0,host=127.0.0.1,port=9001,server=on,wait=off \
       -chardev socket,id=compare0-0,host=127.0.0.1,port=9001 \
       -chardev socket,id=compare_out,host=127.0.0.1,port=9005,server=on,wait=off \
       -chardev socket,id=compare_out0,host=127.0.0.1,port=9005 \
       -object filter-mirror,id=m0,netdev=hn0,queue=tx,outdev=mirror0 \
       -object filter-redirector,netdev=hn0,id=redire0,queue=rx,indev=compare_out \
       -object filter-redirector,netdev=hn0,id=redire1,queue=rx,outdev=compare0 \
       -object iothread,id=iothread1 \
       -object colo-compare,id=comp0,primary_in=compare0-0,secondary_in=compare1,\
    outdev=compare_out0,iothread=iothread1 \
       -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
    children.0.file.filename=$imagefolder/primary.qcow2,children.0.driver=qcow2 -S

2. Secondary: Active and hidden images need to be created only once and the size should be the same as `primary.qcow2`. Again, you don't need to change any IP's here, except for the `$primary_ip` variable:

    # imagefolder="/mnt/vms/colo-test-secondary"
    # primary_ip=127.0.0.1

    # qemu-img create -f qcow2 $imagefolder/secondary-active.qcow2 10G

    # qemu-img create -f qcow2 $imagefolder/secondary-hidden.qcow2 10G

    # qemu-system-x86_64 -enable-kvm -cpu qemu64,kvmclock=on -m 512 -smp 1 -qmp stdio \
       -device piix3-usb-uhci -device usb-tablet -name secondary \
       -netdev tap,id=hn0,vhost=off,helper=/usr/lib/qemu/qemu-bridge-helper \
       -device rtl8139,id=e0,netdev=hn0 \
       -chardev socket,id=red0,host=$primary_ip,port=9003,reconnect-ms=1000 \
       -chardev socket,id=red1,host=$primary_ip,port=9004,reconnect-ms=1000 \
       -object filter-redirector,id=f1,netdev=hn0,queue=tx,indev=red0 \
       -object filter-redirector,id=f2,netdev=hn0,queue=rx,outdev=red1 \
       -object filter-rewriter,id=rew0,netdev=hn0,queue=all \
       -drive if=none,id=parent0,file.filename=$imagefolder/primary.qcow2,driver=qcow2 \
       -drive if=none,id=childs0,driver=replication,mode=secondary,file.driver=qcow2,\
    top-id=colo-disk0,file.file.filename=$imagefolder/secondary-active.qcow2,\
    file.backing.driver=qcow2,file.backing.file.filename=$imagefolder/secondary-hidden.qcow2,\
    file.backing.backing=parent0 \
       -drive if=ide,id=colo-disk0,driver=quorum,read-pattern=fifo,vote-threshold=1,\
    children.0=childs0 \
       -incoming tcp:0.0.0.0:9998

3. On Secondary VM's QEMU monitor, issue command:

    {"execute":"qmp_capabilities"}
    {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "return-path", "state": true }, {"capability": "x-colo", "state": true } ] } }
    {"execute": "nbd-server-start", "arguments": {"addr": {"type": "inet", "data": {"host": "0.0.0.0", "port": "9999"} } } }
    {"execute": "nbd-server-add", "arguments": {"device": "parent0", "writable": true } }

Note:

  1. The qmp command `nbd-server-start` and `nbd-server-add` must be run before running the qmp command migrate on primary QEMU

  2. Active disk, hidden disk and nbd target's length should be the same.

  3. It is better to put active disk and hidden disk in ramdisk. They will be merged into the parent disk on failover.

4. On Primary VM's QEMU monitor, issue command:

    {"execute":"qmp_capabilities"}
    {"execute": "blockdev-add", "arguments": {"driver": "nbd", "node-name": "nbd0", "server": {"type": "inet", "host": "127.0.0.2", "port": "9999"}, "export": "parent0", "detect-zeroes": "on"} }
    {"execute": "x-blockdev-change", "arguments":{"parent": "colo-disk0", "node": "nbd0" } }
    {"execute": "migrate-set-capabilities", "arguments": {"capabilities": [ {"capability": "return-path", "state": true }, {"capability": "x-colo", "state": true } ] } }
    {"execute": "migrate", "arguments": {"uri": "tcp:127.0.0.2:9998" } }

Note:

  1. There should be only one NBD Client for each primary disk.

  2. The qmp command line must be run after running qmp command line in secondary qemu.

5. After the above steps, you will see, whenever you make changes to PVM, SVM will be synced. You can issue command `{ "execute": "migrate-set-parameters" , "arguments":{ "x-checkpoint-delay": 2000 } }` to change the idle checkpoint period time

### Failover test

You can kill one of the VMs and Failover on the surviving VM:

If you killed the Secondary, then follow "Primary Failover". After that, if you want to resume the replication, follow "Primary resume replication"

If you killed the Primary, then follow "Secondary Failover". After that, if you want to resume the replication, follow "Secondary resume replication"

#### Primary Failover

The Secondary died, resume on the Primary:

    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "child": "children.1"} }
    {"execute": "blockdev-del", "arguments": {"node-name": "nbd0"} }
    {"execute": "object-del", "arguments":{ "id": "comp0" } }
    {"execute": "object-del", "arguments":{ "id": "iothread1" } }
    {"execute": "object-del", "arguments":{ "id": "m0" } }
    {"execute": "object-del", "arguments":{ "id": "redire0" } }
    {"execute": "object-del", "arguments":{ "id": "redire1" } }
    {"execute": "x-colo-lost-heartbeat" }

#### Secondary Failover

The Primary died, resume on the Secondary and prepare to become the new Primary:

    {"execute": "nbd-server-stop"}
    {"execute": "x-colo-lost-heartbeat"}

    {"execute": "object-del", "arguments":{ "id": "f2" } }
    {"execute": "object-del", "arguments":{ "id": "f1" } }
    {"execute": "chardev-remove", "arguments":{ "id": "red1" } }
    {"execute": "chardev-remove", "arguments":{ "id": "red0" } }

    {"execute": "chardev-add", "arguments":{ "id": "mirror0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9003" } }, "server": true } } } }
    {"execute": "chardev-add", "arguments":{ "id": "compare1", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "0.0.0.0", "port": "9004" } }, "server": true } } } }
    {"execute": "chardev-add", "arguments":{ "id": "compare0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": true } } } }
    {"execute": "chardev-add", "arguments":{ "id": "compare0-0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9001" } }, "server": false } } } }
    {"execute": "chardev-add", "arguments":{ "id": "compare_out", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": true } } } }
    {"execute": "chardev-add", "arguments":{ "id": "compare_out0", "backend": {"type": "socket", "data": {"addr": { "type": "inet", "data": { "host": "127.0.0.1", "port": "9005" } }, "server": false } } } }

#### Primary resume replication

Resume replication after new Secondary is up.

Start the new Secondary (Steps 2 and 3 above), then on the Primary:

    {"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.2:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }

Wait until disk is synced, then:

    {"execute": "stop"}
    {"execute": "block-job-cancel", "arguments":{ "device": "resync"} }

    {"execute": "blockdev-add", "arguments": {"driver": "nbd", "node-name": "nbd0", "server": {"type": "inet", "host": "127.0.0.2", "port": "9999"}, "export": "parent0", "detect-zeroes": "on"} }
    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "nbd0" } }

    {"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
    {"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
    {"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }

    {"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
    {"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.2:9998" } }

Note: If this Primary previously was a Secondary, then we need to insert the filters before the filter-rewriter by using the ""insert": "before", "position": "id=rew0"" Options. See below.

#### Secondary resume replication

Become Primary and resume replication after new Secondary is up. Note that now 127.0.0.1 is the Secondary and 127.0.0.2 is the Primary.

Start the new Secondary (Steps 2 and 3 above, but with primary_ip=127.0.0.2), then on the old Secondary:

    {"execute": "drive-mirror", "arguments":{ "device": "colo-disk0", "job-id": "resync", "target": "nbd://127.0.0.1:9999/parent0", "mode": "existing", "format": "raw", "sync": "full"} }

Wait until disk is synced, then:

    {"execute": "stop"}
    {"execute": "block-job-cancel", "arguments":{ "device": "resync" } }

    {"execute": "blockdev-add", "arguments": {"driver": "nbd", "node-name": "nbd0", "server": {"type": "inet", "host": "127.0.0.1", "port": "9999"}, "export": "parent0", "detect-zeroes": "on"} }
    {"execute": "x-blockdev-change", "arguments":{ "parent": "colo-disk0", "node": "nbd0" } }

    {"execute": "object-add", "arguments":{ "qom-type": "filter-mirror", "id": "m0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "tx", "outdev": "mirror0" } }
    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire0", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "indev": "compare_out" } }
    {"execute": "object-add", "arguments":{ "qom-type": "filter-redirector", "id": "redire1", "insert": "before", "position": "id=rew0", "netdev": "hn0", "queue": "rx", "outdev": "compare0" } }
    {"execute": "object-add", "arguments":{ "qom-type": "iothread", "id": "iothread1" } }
    {"execute": "object-add", "arguments":{ "qom-type": "colo-compare", "id": "comp0", "primary_in": "compare0-0", "secondary_in": "compare1", "outdev": "compare_out0", "iothread": "iothread1" } }

    {"execute": "migrate-set-capabilities", "arguments":{ "capabilities": [ {"capability": "x-colo", "state": true } ] } }
    {"execute": "migrate", "arguments":{ "uri": "tcp:127.0.0.1:9998" } }

Copyright (c) 2016 Intel Corporation
Copyright (c) 2016 HUAWEI TECHNOLOGIES CO., LTD.
Copyright (c) 2016 Fujitsu, Corp.
Copyright (c) 2026 Lukas Straub <lukasstraub2@web.de>